corresponds to the sum of the Feynman diagrams which are obtained by means of the Wick contractions [1]} of the fields. The set of the connected diagrams is denoted by \(\left\langle \, {\cal P} [\phi ] \, e^{i S_I [\phi ]} \, \right\rangle _0^c\) . The Feynman propagator reads

\(\langle 0 | \, {\rm T} \left( \phi (x) \, \phi (y) \right) | 0 \rangle =\mathop {\vbox {{#\crcr {\hspace{3.0pt}}\,\,\vrule depth4pt width0.4pt\hfill \vrule depth4pt width0.4pt\,\,\crcr {\hspace{0.8pt}}\hfil \displaystyle {\phi (x) \, \phi }\hfil \crcr }}}\limits (y) = i \, \langle x | \, \frac{1}{-\partial ^2 - m^2 + i \epsilon } \, | y \rangle \; .\) 
